The present invention relates to the construction of large cold roll forming machines (hereinafter frequently referred to as "corrugator") such as are needed, for example, to produce the 6" deep.times.16" pitch corrugations employed in the bridge constructions disclosed and claimed in the recently issued U.S. Pat. Nos. 4,120,065 and 4,129,917.
In the past, such deep corrugations have normally been formed in large press-breaks, if they could be formed at all, as single pitch corrugated sections, i.e. U-shaped sheet metal sections defining just one corrugation and which had a length limited by the effective length of the press-break.
Up to the present the size and cost of prior art corrugators capable of cold rolling corrugated sheet metal sections having multiple corrugations of the above discussed large size were considered technically and economically impractical or unfeasible. For example, to cold roll a trapezoidal section with multiple 6" deep corrugations in accordance with the prior art would require a corrugator in which each successive stand corrugates the section at most from about 1/16 to 1/8" deep. Thus, to produce a 6" deep section in 1/8 increments would require 48 stands or more. Such a machine would be prohibitively expensive to construct, operate and maintain, and it would further require a huge and therefore very expensive building.
Since the initial cost of a corrugator is generally very high and their capacity is very large, only few, if any, manufacturers can use a given machine full time for the production of only a single profile. Accordingly, it has been common to purchase such corrugators with additional tooling so that they can produce a variety of profiles and so as to render them economically more efficient. In order to effectively utilize the additional tooling the machine must allow a fast changeover of the tooling from one profile to the next so as to maintain the machine downtimes as short as possible.
However, if a machine is constructed large enough to produce the above mentioned large corrugation profile (6".times.16"), particularly in heavier metal thicknesses, the size of the rolling dies becomes very large and they become correspondingly heavy. Consequently, to change the dies of such machines from one profile to the next would not only require the disassembly of an inordinate number of rolling stands but further would require the handling of individual rolling dies which might weigh several tons each. Thus, the changeover for such a machine for rolling differing profiles becomes a major, time-consuming and, therefore, expensive and self-defeating task. These and other problems connected with cold rolling the large size corrugations renders corrugators constructed in accordance with the prior art unsuitable.
The size of any given rolling or corrugating stand is, of course, determined by the maximum size of the corrugation that is desired to be rolled, the degree of deformation that takes place at a particular stand and, therefore, the forces applied against the rolls as the metal sheet passes the stand, and the amount by which the sheet is deflected in each stand. One apparent way to reduce the enormous size of a corrugator constructed in accordance with the prior art is to increase the amount of metal deformation that takes place at each stand. This, however, may cause the sheet to wrinkle, crack or the like, and requires the transmission of forces between the sheet and the rolls which can become very large as a result of the increased metal deformation that takes place at any given stand. Although the roll itself can be strengthened, for example, by increasing the roll and shaft diameters, a limiting factor is frequently a limit in the transmission of forces from the power-driven or power-rotated rolls to the sheet since such forces can only be transmitted by friction.
Ideally, the speed of each roll equals the speed of the sheet past the corrugating stand. In such an event there is no slippage and a maximum transmission of forces. Practically, however, this cannot take place because in profile the corrugating rolls have varying diameters. In the above example of a 6".times.16" corrugation profile the rolls have maximum and minimum diameters which differ by 12". When the rolls are power-driven the peripheral or surface speeds between the largest and the smallest roll diameters may vary by as much as 20% or more. Thus, in actuality the peripheral speed of the rolls equals the speed of the sheet past the corrugating stands at only one roll diameter. At all other diameters of the rolls the peripheral speeds differ from the speed of the sheet.
The differences between the surface speeds of the sheet and of the rolls results in slippage; portions of the sheet will typically travel faster than parts of the rolls and other portions will travel slower. For conventional, shallow, e.g. 1" to 2" deep corrugations and relatively thin plates such slippage can be tolerated, if necessary by adding corrugation stands and thereby reducing the forces that are applied at each of them. However, for deep corrugations and/or thick plates slippage makes it difficult if not impossible to effectively transmit the large forces that are required to corrugate the sheet in large increments and in a relatively few corrugating stands. Even more seriously, the relative motion between the rolls and the sheet, coupled with the necessary large forces would subject the rolls to extreme wear and galling, thereby not only damaging the rolls and rendering very expensive equipment useless but further damaging the surface of the sheet.
Additionally, when portions of the sheet travel faster and other portions slower than corresponding portions of the rolls, the top and bottom portions of the corrugated sheet, i.e. the corrugation peaks and troughs are subjected to severe forces and resulting bending moments caused by this difference in the relative speeds between the sheets and the rolls. These forces and moments tend to warp and bend the sheet and they can skew it, that is deflect it from a straight travel path perpendicular to the axes of the rolls to one or the other side. Moreover, the relatively weak corrugation sides interconnecting the corrugating peaks and troughs can become wrinkled, resulting in the so-called "oil canning effect", particularly when the sheet is relatively thin. Thus, a sheet corrugated in this manner is likely to be warped, dimensionally inaccurate, wrinkled and, for many applications, useless.
In a typical example for forming 6".times.16" trapezoidal corrugations the neutral axis of the sheet and of the roll dies is spaced 3" from the top and bottom flanges of the corrugated sheet. Assuming the neutral axis of the sheet to travel linearly at a speed of 50' per minute and a roll die having an outermost (peak) diameter of 30" and an opposing innermost (trough) diameter of 18", the peripheral speed of the crown diameter is 62.50' per minute or 25% faster than the peripheral speed of the neutral axis of the roll. Similarly, the trough diameter of the roll die has a peripheral speed of 37.50' per minute or 75% of the peripheral speed of the peak diameter of the roll. In other words, at the stated diameters there is an approximately 67% difference in the peripheral speed between the maximum and the minimum die diameters. This, therefore, results in a 67% relative speed differential between the maximum and minimum roll die diameters and the sheet being corrugated.
Such a difference is too large for producing an acceptable product free of galled surfaces, wrinkles and the like. The difference in peripheral speeds is especially critical when producing trapezoidal patterns (as opposed to making a sinusoidal pattern where the male dies do not need an opposing female die to produce the correct radius at the crowns), since a male or peak die for a trapezoidal pattern must cooperate with a corresponding, opposing female or trough die in order to exert the relatively much greater forces and form the necessary, sharply radiused corners between flat crown or trough sections of the corrugation and the interconnecting slanted corrugation sides.
The forces and particularly the moments generated by the speed differences not only adversely affect the dimensional stability and ultimate shape of the corrugated sheet but, in addition, they build up forces which must be borne by the structural frame of the corrugator, the bearings, the dies and the like which in turn requires that these be given sufficient strength to withstand such forces. This, in turn, increases the overall cost of the corrugator.
As has been demonstrated in the earlier referenced, recently issued U.S. patents, by carefully analyzing and selecting the actual dimensioning of corrugated plate significant advantages can be attained in the strength, weight and cost of the ultimate structure into which the corrugated plate is assembled or installed. To mention a few parameters, the base width of the corrugation peaks and valleys can be varied by slight amounts so as to enable a true nesting of corrugated sheets which facilitates both the ultimate use of the sheet and their storage and shipment by minimizing the volume occupied by a stack of such sheets. Further, the relative width of the corrugation peaks and/or crowns can be varied to maximize the section modulus or moment of inertia of a given structure while minimizing its weight. For similar considerations it is frequently desirable to vary the metal thickness of various corrugated plate components in a corrugated plate structure.
Each time any one of these parameters of a corrugated plate is changed, it is necessary to correspondingly change the roll dies for the plate since effective rolling and an accurate dimensioning of the corrugated plate requires that the spacing between the opposing dies of every corrugating stand substantially equals the desired profile of the plate at the particular stand. This is especially true for the last two stands which determine the ultimate dimensioning and shape of the plate.
Each time the shape of the profile is changed, whether or not this involves a change in the corrugation pitch, it is necessary to install an entirely new set of roll dies in corrugators constructed in accordance with the prior art. This can even be true when the only change is the plate thickness since a mere increase in the spacing between cooperating dies does not correctly change the spacing between the die peripheries. To illustrate the point, if the plate thickness is increased by a given amount and the spacing of cooperating dies is increased by the same amount, the correct distance between the die peripheries can only be obtained for some portions of the corrugated plate, say at the flat and parallel corrugation peaks and troughs. The spacing between the die peripheries for the slanted corrugation sides will be greater than necessary for the changed plate thickness. This, in turn, permits portions of the plate to be loose as it passes between the dies which contributes to dimensional instabilities in the finish corrugated plate.
Thus, for prior art corrugators it is necessary to provide a separate set of dies for each contemplated corrugation profile. The same applies for each contemplated metal thickness although there one can compromise to a certain degree as briefly outlined in the preceding paragraph if one is willing to accept a degree of dimensional instability of the finished product. However, it is apparent that a large number of roll dies are necessary if one desires to take advantage of efficient corrugated plate forms and dimensions. This greatly increases the initial tooling cost for the corrugator and further greatly increases its operating costs because of the need to change heavy dies each time a different profile and/or a different plate thickness is being corrugated.
Apparently, as a result of these large costs, it has heretofore not been the practice to design corrugated plate shapes and to dimension the plate to optimize the efficiency of such plate in the ultimate structure into which it is assembled or installed. Instead, manufacturers have simply manufactured one or two standard plate profiles and dimensions and offered these for sale. It was then left to the engineer to incorporate these profiles to the best of his ability, knowing that he had to compromise structural efficiency of the profiles in order to obtain them at an economically feasible cost.